Add Chrome for Testing support for reliable WebDriver automation

- Add setup script (scripts/setup-chrome-for-testing.sh) that downloads
  matching Chrome and ChromeDriver versions from Google's CDN
- Add chrome_binary config option to specify custom Chrome binary path
- Update ChromeDriver to support custom binary via with_port_headless_and_binary()
- Update README with Chrome for Testing setup instructions
- Update config.example.toml with chrome_binary documentation

Chrome for Testing is Google's dedicated browser for automated testing
that guarantees version compatibility with ChromeDriver, avoiding the
common 'version mismatch' errors when Chrome auto-updates.

crates/g3-config/src/lib.rs

@@ -132,6 +132,10 @@ pub struct WebDriverConfig {
     #[serde(default)]
     pub chrome_port: u16,
     #[serde(default)]
+    /// Optional path to Chrome binary (e.g., Chrome for Testing)
+    /// If not set, ChromeDriver will use the default Chrome installation
+    pub chrome_binary: Option<String>,
+    #[serde(default)]
     pub browser: WebDriverBrowser,
 }
 
@@ -152,6 +156,7 @@ impl Default for WebDriverConfig {
             enabled: true,
             safari_port: 4444,
             chrome_port: 9515,
+            chrome_binary: None,
             browser: WebDriverBrowser::ChromeHeadless,
         }
     }